In the field of flat-panel display devices as typified by liquid crystal display devices and EL (electroluminescence) display devices, in recent years, in order to achieve panel frame narrowness and reduced panel thickness, a so-called driver-integrated display device has been developed in which, on the same transparent substrate with a display unit composed of pixels arranged in a matrix, a peripheral driving circuit for driving the display unit is provided in an integrated form. Since TFTs are used as pixel transistors in liquid crystal display devices and EL display devices, regarding mounting of the driving circuit on the transparent substrate, the driving circuit is also formed by TFTs.
As the peripheral driving circuit for the display device, an analog buffer circuit is used in many cases in order to increase a driving capability. Here, the case of using TFTs to form the analog buffer circuit on the insulating substrate is considered. In TFTs, the absolute value of threshold value Vth is large and its variation is greatly large. Also, it is known that, when TFTs are formed on an insulating substrate such as a glass substrate, the element characteristics of the TFTs are worse than those in the case of TFTs formed on a silicon substrate. As described above, since the absolute value of threshold value Vth of the TFT is large and its variation is greatly large, when TFTs are used to form an analog buffer circuit on the insulating substrate, input and output offsets of the analog buffer circuit and their variations increase.
It is assumed that a plurality of the analog buffer circuit be arranged for, for example, an output unit of a data driver (horizontal driver) in a driver-integrated display device so as to correspond to data lines (signal lines) in the display unit. In this case, large variations in the input and output offsets of the analog buffer circuit cause a large output-potential error in each circuit. Then, this output-potential error is a potential difference between columns of the display unit, and is displayed as a vertical stripe on the screen. Accordingly, this greatly deteriorates display quality (uniformity).
Also, in the case of using TFTs to form an analog circuit on an insulating substrate, a large absolute value of TFT threshold value Vth causes large power consumption because a high power-supply voltage and a large idling current are required for circuit driving. Accordingly, this is a disadvantage in reducing power consumption of the display device, although a driver-integrated configuration is established.
In view of the above problems, the present invention is made and an object thereof is to provide an analog buffer circuit in which, even if the analog buffer circuit is formed by TFTs on an insulating substrate, it has small input and output offsets and less power consumption, a display device in which the analog buffer circuit is used as a peripheral driving circuit for a display unit, and a portable terminal provided with the display device as a screen display unit.